A method is known that is used by the receiving side to extend the frequency band of a speech signal, which is encoded and reproduced at a low-bit rate, without transmitting auxiliary information for band extension from the sending side (for example, Non-Patent Document 1).
Non-Patent Document 1: P.Jax, P.Vary, “Wideband extension of telephone speech using hidden markov model,” Proc. IEEE Speech Coding Workshop, pp. 133-135, 2000
According to the conventional method described in Document 1 described above, the receiving side uses an HMM (Hidden Markov Model) to search for filter coefficients after band extension.
On the other hand, there has been no transcoder that performs inter-conversion between a code encoded in accordance with a first encoding method and a code encoded in accordance with a second encoding method by extending the frequency band of a signal before conversion when converting from a first code to a second code.
The conventional method described in the document described above (Non-Patent Document 1 described above) by P.Jax and P.Vary, which requires the spectrum envelope of a wideband speech and the HMM-based modeling of filter coefficients, has the following problems.
That is, the HMM model parameters must be determined offline from a large-volume speech database in advance, and this processing requires long computation time and high costs.
In addition, the receiving side where the band is extended in real time must perform HMM-model-based search processing that requires a large amount of computation.